I have been flying regularly between La Habana and Miami and the charter flights to Cuba is a different experience.
First of all, almost all passengers would be Cuban Americans going home for a visit and taking large amount of luggage, in fact that is how these charter companies make their money, 2 dollars per pound over anything over 44 lbs in total luggage. also they would charge you 50 dollars us tax which is normally included in the ticket but they may not. if you dont have visa that would be another 50 dollars for the tourist card which is only normally 20 usd.
Recently Cuba has opened up and there are many charter flights but the character of them wouldnt change until commercial flights are in place, ojala, in the future.
Dont expect anything, occasionally if it is a larger plane and since i fly frequently they would upgrade me if i know the agent in Miami but in La habana expect nothing. In flight service would be minimal, perhaps it is the shortness of the flight from Miami from ORD you may have a full service. But remember this is a charter service and their objective is to make as much money as possible.
You will not arrive in the International Terminal but in Terminal 2 which is reserved for cuban americans come home and the immgration can be slow and if you are singled out for customs inspection, that can take some time. all in all cuban officials are present but remember you represent a country which has not been all that good to them, so they are right to hold some resentment against gringos coming to visit. Having said that an average cuban loves USA and you are visiting one of the nicest places to go, dont expect too many amenities but the warmth of the people would make up for it.
I am in the USA at the moment and will be flying out of Miami in a few days time to get back to La Habana and the academic work.
Best wishes
Yehuda